The present invention relates to a method for forming a green body of a ceramic arc tube used in metal halide lamps, and more particularly, the present invention relates to a method for forming a green body of a shaped ceramic arc tube for use in a metal halide lamp.
It is known in the prior art to produce ceramic arc tubes for metal vapor discharge lamps by sealing a tubular translucent alumina-based element that is opened at both ends with heat resisting metal or ceramic caps and by sealing discharge electrodes to central holes of the caps. Production of arc tubes constructed in this manner is complicated and the arc tubes have limited life and stability, because the seal between the caps and the tubular element breaks down over time, due to the poor corrosion resistance of the material used to seal the caps to the tubular element. Additionally, the luminous efficiency and color rendition is not optimal in a ceramic arc tube that has a straight tubular shape.
Integrally shaped ceramic arc tubes in which an outer diameter of an arc discharge portion is larger than that of electrode holding end portions has been proposed in the prior art. U.S. Pat. No. 4,451,418 discloses a ceramic green arc tube for a metal vapor discharge lamp where an outer diameter of the arc discharging portion is larger than that of the end portions that hold discharging electrodes. The ceramic green arc tube is produced by preparing a stiff plastic body that consists mainly of a ceramic material and a binder. The stiff plastic body is formed into a straight tubular body by means of an extruder. The tubular body is placed in an inner cavity of a molding dye that has the shape of the desired ceramic arc tube. One end of the formed stiff plastic tubular body is closed and a compressed fluid is applied to the open end of the tubular body. The extruded tubular body is inflated until a central portion of the tubular body contacts an inner surface of the molding die. The inflated body is hardened and dried with the heat of the previously heated molding dye. A ceramic green arc tube is ejected from the mold.
U.S. Pat. No. 4,387,067 discloses a ceramic arc tube of a metal vapor discharge lamp that has a discharge portion with electrode holding end portions integrally formed at opposite ends thereof. The outside diameter of the arc discharge portion is larger than that of the electrode holding end portions. The ceramic arc tube is made by placing a tubular green body in a fusiform cavity of a die, inflating the middle portion of the green body more than the end portions of the green body, and firing the shaped green body to produce a ceramic arc tube.
The present invention concerns a method for forming a green ceramic arc tube for a metal halide lamp. A feedstock material is prepared by mixing alumina with a binder. The feedstock material is injected into an inner cavity of a mold. The inner cavity of the mold has an inner surface that corresponds to a desired outer shape of a body of the ceramic arc tube. An outer diameter of an arc discharging portion of the desired ceramic arc tube body is larger than end portions of the arc tube that hold discharging electrodes. A fluid is injected into the feedstock material to create a cavity in the feedstock material and to force the feedstock material into contact with the inner surface of the mold. The mold is then separated from the formed ceramic green arc tube.
The feedstock material may be comprised of approximately 80% alumina suspended in a binder comprising approximately 18% carnauba wax and 2% stearic acid by weight. The feedstock material may be heated before being injected into the mold to melt the wax and decrease its viscosity. The fluid used to create a cavity in the feedstock and force the feedstock material into contact with an inner surface of the mold, may have a viscosity that is less than the feedstock material. The ratio of viscosity of the feedstock material to the viscosity of the fluid injected to create the cavity may be over 100 to 1. The injected fluid may be a liquid, such as water or a gas such as Nitrogen. Depending on the type of feedstock material, the feedstock material may be heated or cooled after the fluid is injected into it to increase the viscosity and strength of the feedstock material to allow the formed arc tube to be removed from the mold.
The apparatus used to form a green ceramic arc tube according to the method of the present invention includes a mold, a ceramic feedstock injector and a fluid injection unit. The mold has an inner cavity with an inner surface that corresponds to the desired outer surface of the arc tube. The mold may also include a pin that extends into the inner cavity which defines an inner diameter of an end portion of the arc tube. The mold may further include an injector pin coupled to the fluid injector for injecting fluid into the ceramic feedstock material. The mold may include a core pull mechanism for removing the arc tube from the mold. The mold may be comprised of two sections that have opposing surfaces that are transverse to an axis that extends through the cavity and end portions of the formed arc tube. The ceramic feedstock injector has an outlet coupled to a feedstock inlet in the mold. The ceramic feedstock injector is adapted to inject a ceramic feedstock into the mold. The fluid injection unit has a fluid outlet coupled with a fluid inlet of the mold. The fluid outlet of the fluid injection unit may be coupled to an injector pin that injects a fluid into the ceramic feedstock.
The single step process of the present invention allows arc tubes to be produced with significant material and process time savings. Wall thickness distribution can be tailored by varying the heat transfer and rheology of the process. The present invention allows arc tubes to be made in a variety of shapes and sizes with reduced cycle times. The walls of the arc tubes are more tightly packed by exerting pressure through the fluid, which results in fewer defects in the arc tubes.